Week 7 Flashcards
What is the spin ? What is the strength of a magnatic dipole and how can it align with B?
Spin is a quantum mechanical property of a particle, here of a nucleus.
Strength = gyromagnetic ratio
It can align parallel or antiparallel
what is the Nuclear Zeeman effect ?
When B>0, the parallel and antiparallel spins separate and go on different energy levels
What happens if a photon is absorbed, or emitted? What is the Larmor frequency and the formula?
Low energy proton can absorb energy and jump to higher spin level (or opposite).
Larmor frequency is frequency of incoming / emitted photon: w = gyromagnetic ratio * B [rad]
In what range are the frequencies?
radiofrequency range
list a few common nucleis for NMR
- 1H: most common, strongest signal (highest GMR)
- 13C: low natural abundance
- 19F: strong signal, but not natural in tissues
- 31p
What is the population of energy levels at 0K and 300K ? consequence ?
0K : all in the ground state
300K : only a very small population difference between high and low energy -> NMR is a pretty insensitive technique
What is the effect of the oscillating field B1 ? What happens during relaxation?
magnetization turns away from initial orientation, and rotates at Larmor frequency.
Then it returns to initial alignment with B0
What does rotating M induce ? How is this signal processed?
voltage in receiver coil -> this is our signal
Fourier transform, broader peak because of free induction decay (caused by relaxation)
How many times is the pulse sequency usually done? How has the SNR changed?
when the signal is added 4 times, the SNR doubles
name three important things for the spectrometer (the machine)
- superconducting magnet for B0
- needs to be cooled with liquid helium (4.2K)
- magnet always on
what is the chemical shift ? how is it related to shielding?
shift of the spectrum position because of environmental effects.
Shielding = cloud of electrons of surrounding atoms that produce an induced Bi opposed to B0
- less shielded = higher f (more to the left)
- more shielded = lower f (more to the right)
how do you calculate the chemical shift in ppm ?
shift in Hz left of TMS / spectrometer frequency in MHz
what is J coupling? Does this depend on B0?
Spin of one nuclei affects spin of different nuclei via bonds (homonuclear or heteronuclear)
INDEPENDENT of B0!
n+1 rule
What is better with high field strength?
Higher signal.
Better separation of multiplets.
name 5 pros and 5 cons of NMR
- many sample types
- wide range of info
- non-destructive, non-invasive
- easy sample prep
- high reproducibility of spectra
- only nuclei with spin
- low sensitivity
- expensive hardware and maintenance
- artifacts caused by impurities, B inhomogeneities
- data analysis can be complex
